in Img3D::Tool::insideForm, use generic algorithm to determine whether a point is inside a polyhedron

Img3D::Tool::insideForm is used in visualizing a mesocrystal, to determine whether or not a lattice point is to be displayed.

To save separate code for each polyhedral shape, we should use a generic algorithm, to be implemented in libformfactor, libformfactor#1 (closed).